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Many things exist. Everything that exists interacts with every other thing that exists, and no matter how small or attenuated that interaction may be it is not zero. For an existent to be somehow isolated fully from every aspect of existence it would effectively be in its own separate universe, unknowable and epistemologically out-of-bounds as an object of valid thought.
Identity which does not involve a relational aspect with other identities is just unknowable. So it can't be discussed.
Metaphysics and epistemology go together because the limits of what can can be claimed to exist coincide with the limits of what is knowable. No one can justifiably confirm or deny either the existence or nonexistence of what is outside of the Universe. Any justification that one might discover to such an isolated unknown would also be a casual link that would rope that existent into inclusion in what the concept Universe refers to which is the entirety of existence. Existence is Identity is Casuality.